RPC_AUTH_MAXFLAVOR is an invalid flavor, on purpose. Don't allow
any processing whatsoever if a caller passes it to rpcauth_create()
or rpcauth_get_gssinfo().
Signed-off-by: Chuck Lever <chuck.lever@oracle.com> Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>